Last July I was out in the vegetable garden, when a neighbor gardener approached and asked, “What are you doing?” I responded, “Smashing Mexican bean beetle larvae.” My neighbor gave me a puzzled look, followed by a smirking smile, as if I was committing a gardening no-no. I just had to ask, “You’re not having a beetle problem on your green beans?” He smiled and said, “Of course not I plant marigolds with my beans, and they keep the bugs out of the bean patch.” WOW! Could this be the silver bullet in controlling this obnoxious pest?
Now that the gardening season is over and the cover crop is planted, I’ve got a chance to catch my breath from sowing, planting, mulching, weeding and harvesting. Did I mention weeding? Anyway, now that I have a little time to reflect back on the garden season as to what may or may have not worked well, I’ve been returning to that conversation with my neighbor about the wonders of the marigold. The seed catalogs are starting to show up in the mail, and the planning for next year’s garden is in the beginning stage, so before ordering a ton of marigold seeds, now would be a good time to do a little research on the magical repelling powers of the marigold plant. I’ve heard marigold stories for years, how marigolds will repel every garden pest known to mankind, including bugs, snails, rabbits, ground hogs, and deer! I’ve even heard that it has been used to target and kill selected weeds. This sunny annual has been employed as a companion plant for generations just to do that — repel pests from the garden. Do they really benefit the garden as a repelling machine or are they just pretty and their repelling powers just a gardening myth?
The Plant
Marigolds belong to the aster family (Asteraceae), genus Tagetes. Their natural range extends from the southwestern United States into Argentina, with their greatest distribution being in south central Mexico. Approximately 50 species are known, but in general, the three most common are African marigolds (T. erecta), French marigolds (T. patula) and Signet marigolds (T. tenuifolia). However, regardless of their name, all marigolds are native to subtropical America and have been cultivated in Mexico for over 2,000 years.
Tagetes patula, commonly called French marigold, is a compact annual that typically grows 6-12” tall and features single, semi-double, double or crested flowers (1-2” diameter) in shades of yellow, orange, red and bicolor. Their pinnate leaves with toothed, lance-shaped leaflets are aromatic.
Tagetes erecta, commonly called African marigold, Aztec marigold, American marigold or big marigold, is native to Mexico and Central America. Big marigold may be the most descriptive of its names because plants are noted for their large flowerheads. They typically grow from 1-4’ tall and feature huge, mostly double-globular flowers (2-4” diameter) in various shades of yellow, orange, and white. Foliage and flowers are aromatic when brushed or crushed. Triploid F1 hybrids (T. erecta x T. patula) combine the large flowers of the African marigold with the more compact size of the French marigold into vigorous plants with 2-3” diameter flowers on stems reaching 10-18” tall. These triploids are largely unaffected by high heat and usually bloom all summer.
Tagetes tenuifolia. Signet marigolds are compact, mounding plants with smaller flowers and leaves than most other marigolds. Yellow, orange, golden, or bicolored flowers are held either well above the fine-textured, dark green foliage or tucked in with the foliage, depending on the cultivar. This plant doesn’t have that overwhelming marigold scent but has a light, citrusy smell.
Fact or Fiction
For generations, many vegetable gardeners have planted marigolds in their vegetable patches to repel pests. However, there is a lack of scientific evidence to support the notion that marigolds actually repel pests. Research conducted at Rutgers University concluded that marigolds failed to repel cabbage, carrot and onion pests. In fact the USDA lists a total of 15 pests that attack marigolds; included on their list are aphids, Japanese beetles, snails, and spider mites, just to name a few. On the other hand, researchers at the University of Vermont have reported that marigolds have been effective in luring pests away from other ornamental plants.
Although science has yet to prove that marigolds actually repel pests from vegetable crops, there is scientific evidence that marigolds CAN be an important tool in controlling certain nematodes. Nematodes are tiny worms, usually microscopic in size. Nematodes that feed on plants — called plant-parasitic nematodes — have spear-like mouthparts used to puncture plant roots to obtain nutrients. As a result, plant-parasitic nematodes can seriously damage or even kill crops, turf, and ornamental plants.
How marigolds help fight nematodes
Marigold roots release a toxic chemical (alpha-terthienyl), and the presence of this chemical inhibits the hatching of nematode eggs. Therefore, control of the nematode population is achieved by interrupting the nematode life cycle.
One drawback with using marigolds for nematode control is that the benefit is not realized until the following year. To be effective the marigolds must be planted before the vegetable crop — at least 2 months before — and must be planted at the same location where the vegetable crop is to be planted; otherwise, no benefits will be gained from the marigold root exudates. For example, California research showed that tomatoes grown after marigolds had significantly lower numbers of root galls due to root-knot nematodes (Meloidogyne incognito). In addition, the tomato yields were higher (root length, shoot weight, number and weight of fruits were all higher in plants grown after Tagetes). UniversityofCaliforniaRiverside.edu.
Varieties Matter
Care should be taken when purchasing marigolds for controlling nematodes. That’s because “not all marigold varieties control all types of nematodes.” Univ.ofFlaExt.edu/ng045. For example, the California research mentioned above revealed that particular varieties are more effective at controlling root-knot nematodes. In that experiment, the “Single Gold” variety of Tagetes patula outperformed other varieties. You’ll get the best results if you determine which types of nematodes are in your local soils, and you can do this by sending soil samples to a nematode assay laboratory. Univ.ofFlaExt.edu/ng045 (look at the chart in this article identifying marigold species and varieties by their resistance to and effectiveness against particular types of root-knot nematodes).
Attracting Beneficial Insects
In addition to helping control nematodes, marigold flowers attract beneficial insects that not only pollinate, but also help control bad bugs. Beneficial insects attracted to marigolds include: hover flies, lady bugs and parasitic wasps.
Conclusion
To date there is very little scientific evidence that the aroma of the marigold plants actually repel pests, however it is a generally accepted scientific fact that marigolds help to control nematodes and attract beneficial insects that aid in controlling unwanted pests.
There is growing concern about pesticides’ non-target effects on humans and other organisms, and many pests have evolved resistance to some of the most commonly-used pesticides. Together, these factors have led to increasing interest in non-chemical, ecologically sound pest management. The marigold is not only pretty but offers the gardener another arrow for the quiver in the bug war. Who wouldn’t want to plant a beautiful plant that was edged out by the rose for our national flower. I know I will!
Thanks for stopping by The Garden Shed. We members of The Garden Shed Team wish you and your family a safe and happy holiday season.
Sources:
“Tagetes L., Marigold,” Plant Profile, USDA https://plants.usda.gov/core/profile?symbol=TAGET
“Tagetes erecta, ” Missouri Botanical Garden, http://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=277371
“Effects of Selected Marigold Varieties on Root-knot Nematodes and Tomato and Melon Yields”, Department of Namatology, University of California, http://faculty.ucr.edu/~atploeg/PDF PAPERS/PLANT DISEASE/PLANTDISEASE.pdf
“Marigolds (Tagetes spp.) for Nematode Management,” University of Florida Institute of Food and Agricultural Sciences, Univ.ofFlaExt.edu/ng045.
“Companion Planting and Insect Pest Control,” Rutgers University, http://pemaruccicenter.rutgers.edu/assets/PDF/publications/2013-InTech-Companion_planting_and_insect_pest_control.pdf
“Murdering Thrips with Marigolds, Fungi & Mites,” University of Vermont Entomology Research Laboratory, https://www.uvm.edu/~entlab/Greenhouse IPM/Workshops/2015/AFEProjectDesc&SummaryDec2014.pdf
“Root-knot Nematodes: Biocontrol with Marigolds,” North Carolina Cooperative Extension, https://carteret.ces.ncsu.edu/wp-content/uploads/2013/07/Root-Knot-Nematodes-Biocontrol.pdf?fwd=no
“The Repellent Marigold And Other Myths Of Companion Gardening,” The Chicago Triburne, http://articles.chicagotribune.com/1992-09-13/news/9203230490_1_companion-plantings-carrots-love-tomatoes-bedding-plants
“Dispelling Marigold Myths,” Alabama Cooperative Extension System, http://www.aces.edu/dept/extcomm/specialty/marigolds.html
“Parasitic Wasps: A Gardener’s Best Friend,” North Carolina Cooperative Extension, https://brunswick.ces.ncsu.edu/2014/04/parasitic-wasps-a-gardeners-best-friend/
“PROMOTING THE MARIGOLD AS NATIONAL FLORAL EMBLEM,” The Dirksen Congressional Center, http://www.dirksencenterprojects.org/promotingmarigold.pdf
“Controlling Insects on Flowers,” USDA, https://naldc.nal.usda.gov/download/CAT87210014/PDF
2003 Annual Report of Accomplishments and Results, The Connecticut Agricultural Experiment Station, https://portal.nifa.usda.gov/web/areera/Reports/2003/Ct/AES.NewHaven.CT.pdf
“Attracting Beneficial Insects, Penn State University,” https://extension.psu.edu/attracting-beneficial-insects